1. Field of the Invention
This invention relates to a digital AM transmitter obtained by digitizing an AM transmitter for a medium frequency radio broadcast.
2. Description of the Related Art
FIG. 1 shows a typical conventional digital AM transmitter.
In FIG. 1, reference numeral 11 denotes an input terminal for a audio signal A. The signal A is supplied from the input terminal 11 to an A/D converter 12, where the signal is converted to a digital signal of, for example, 12 bits, as indicated by the binary number system (2.sup.0, 2.sup.1, 2.sup.2, . . . 2.sup.11). All the bits of the digital signal are applied to a code unit 13 in a parallel manner.
The code unit 13 has 4095 output terminals (#1-#4095) corresponding to the 12-bit binary signal, and outputs a bit signal of "1" from one of the output terminals determined by the bit value of the bit signal. For example, when the value of "2.sup.0 " bit is 1 (i.e., when the value is 1 in the decimal number system), "1" is outputted to a bit signal output terminal #1. When the value of "2.sup.1 " bit is 1 (i.e., when the value is 2 in the decimal number system), "1" is outputted to two output terminals #2 and #3. Further, when the value of "2.sup.2 " bit is 1 (i.e., when the value is 4 in the decimal number system), "1" is outputted to four output terminals #4, #5, #6 and #7.
In other words, the code unit 13 outputs "1" from output terminals with smaller # numbers when the level of a audio signal is low, and outputs "1" from both output terminals with larger # numbers and those with smaller # numbers when the level of the audio signal is high.
The bit signals outputted from the output terminals #1-#4095 of the code unit 13 serve as switch control signals and are supplied to 4095 carrier wave switches (#1-#4095) 15, respectively.
Reference numeral 17 denotes an input terminal for receiving a carrier wave signal C. The carrier wave signal C supplied to the input terminal 17 is divided into 4095 portions by a carrier wave divider 18 and supplied to the carrier wave switches (#1-#4095) 15, respectively.
Each carrier wave switch 15 is electrically conductive when the switch control signal from the code unit 13 is "1", and nonconductive when the switch control signal is "0", thereby selectively receiving divided portions of the carrier wave signal C. Each of the received portions of the signal C is amplified by a corresponding one of 4095 power amplifiers 16 (#1-#4095) with a predetermined gain, and then supplied to a carrier wave combiner 19.
The carrier wave combiner 19 has 4095 transformers (#1-#4095). The primary winding of each transformer is connected to the output terminal of a corresponding one of the 4095 power amplifiers 16, and the secondary windings of the transformers are connected to one another in series. One end of the connected secondary windings is grounded, and the other end is used as an output terminal.
In the carrier wave combiner 19, constructed as above, when those portions of the carrier wave signal C which have been amplified by the power amplifiers 16 (#1-#4095) are supplied to the corresponding ones of the primary windings, they are combined and digitally summed up on the side of the secondary windings. A combined output is generated from the secondary windings, and is outputted as an amplitude modulated wave signal AM from an output terminal 21 after an unnecessary high frequency component of the combined output is removed by a bandpass filter 20.
The conventional AM transmitter, constructed as above, has a high level of efficiency and performance, but also carries the following drawbacks:
Since the code unit has output terminals corresponding to the quantizing step of an A/C converter, and generates "1" sequentially therefrom, in an order beginning from an output terminal with the smallest number, output terminals with larger numbers generate "1" only when the audio signal has a high level. On the other hand, output terminals with smaller numbers generate "1" when the audio signal has either a high level or a low level.
Therefore, the smaller the number of the output terminal is, the more often "1" is generated. As a result, power amplifiers corresponding to output terminals with smaller numbers operate for longer and hence generate a larger amount of heat than those corresponding to output terminals with larger numbers.
In summary, in the conventional digital AM transmitter, the operation time and the heating value are not uniform between the power amplifiers, and therefore it is possible that some amplifiers will be worn out earlier than others. This means that the power amplifiers cannot have a consistent level of reliability, which is a significant problem in the case of a great power transmitter such as a middle wave radio transmitter.